[Halld-tagger] Tagger field maps

[Richard Jones]

Dan,

I assume you mean in-plane focusing, not vertical focusing which would require out-of-plane field measurements.  It will be interesting to see what you find the sensitivity to be.  The plan you have to test it by interpolating the existing TOSCA map with full and degraded resolution, and tracking electrons through it sounds like it will tell us the sensitivity to the grid spacing at the edge.  Sounds good.

-Richard J.


On 7/18/2013 11:27 AM, Daniel Sober wrote:
> Richard,
> The error in the field integral is hard to quantify -- it depends on how the raytracing codes interpolate between the field points (probably effectively linearly).  In any case, the important question is the effect on the position and direction of the electrons at the focal plane.  A substantial error in B at the exit edge may influence the focusing without having much effect on integral B*L.  I am trying to refresh my knowledge of SNAKE (with the help of John LeRose), and run some tagger orbits through the Tosca field using several grids:  1x1 cm (as provided by Yang), 1x2 cm (close to what we expect to map) and 2x2 cm.  If the 2x2 results are in acceptable agreement with the 1x1 (I would say within +-1/2 microscope fiber is acceptable) then we don't need to worry.
>
> Tim,
> As I look closer at my plots of fields at different magnet excitations, I think we can live with a fairly sparse grid of measurements at fields other than 15 kG, and then fit some kind of smooth ratio function to the 15 kG map.
>
> How long do you anticipate a full 1x2.5 cm map will take, and how much time would be saved by increasing the step size by factors of 2 or 4 in each direction?  Do we plan to measure several field values for each mapper setup before moving the apparatus?
>
> Dan
>
> On 7/17/2013 5:37 PM, Richard Jones wrote:
>> Hi Dan,
>>
>> Welcome back, hope you are doing well.
>>
>> Do you have an estimate of the error in the field integral? We could introduce a bias in the map that zeros out the error in the field integral to leading order, couldn't we?
>>
>> -Richard J.
>>
>>
>> On 7/17/2013 3:26 PM, Daniel Sober wrote:
>>> I'm back from vacation, and looking harder at the Tosca fields.
>>> My worry about the grid size near the exit edge of a 3 cm gap is that almost any interpolation procedure is likely to introduce substantial errors.
>>> The attached plot shows the field vs x at z=0 (magnet center).
>>> The circles are the Tosca calculated values at x = 32, 33, 34, ..., 44 cm (1 cm steps).
>>> The two curves (with symbols + and ×) show cubic spline interpolations (using the Numerical Recipes subroutines) between two different 2 cm grids, corresponding to x = 33, 35, 37, ... cm (odd x) and x = 32, 34, 36, ... cm (even x).  For each integer value of x, one of the two interpolations is necessarily exact, but the other is far off (~1.5%) near the field boundary.
>>> The next step is to compare the results of ray-tracing using a 1-cm grid and a 2-cm grid.  I will try to set this up using SNAKE, which I am familiar with (or at least I was 10 years ago.)
>>> I realize that (1) the errors due to interpolating on a 1-cm grid will be much smaller than on a 2-cm grid, and (2) the number of affected points contribute relatively little to the total field integral, so maybe my worries are for nothing, but this effect has to be quantified.
>>> Dan
>>>
>>>
>>> -- 
>>> /Daniel Sober,
>>> Professor
>>> Physics Department
>>> The Catholic University of America
>>> Washington, DC 20064
>>> Phone: (202) 319-5856, -5315
>>> E-mail: sober at cua.edu/
>>
>
> -- 
> /Daniel Sober
> Professor
> Physics Department
> The Catholic University of America
> Washington, DC 20064
> Phone: (202) 319-5856, -5315
> E-mail: sober at cua.edu/

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